The control of intra-cellular cholesterol levels is an intricately regulated process that depends not only on the regulation of lipid uptake and cellular sterol synthesis but also on a mechanism that permits cells to efflux cholesterol to apolipoprotein and lipoprotein acceptor molecules. The best evidence for the importance of this efflux pathway in human physiology comes from studies of patient's with Tangier disease. Recent work has shown that the genetic defect in Tangier disease maps to a locus encoding a member of the ABCA family of ATP- binding cassette transporters (ABCA1) and that this transporter is required for normal, apolipoprotein-stimulated cholesterol efflux. In vivo, the reverse cholesterol transport pathway is thought to be initiated when a lipid-poor HDL binds to a cell and stimulates the removal of cellular lipids, at least in part, via this ABCA1-mediated process. As the pharmacologic tools for manipulating the reverse cholesterol transport pathway are far less sophisticated and effective than those available for reducing the levels of pro-atherogenic lipoproteins, insights gained from investigating ABCA1's function have the potential to lead to new clinical therapeutics. In this proposal, the investigators will explore the structural requirements that underlie ABCA1 function, focusing on its membrane topology, binding interaction with ligands, and regulation by kinases. These studies of the cell biology of the ABCA1 transporter will importantly contribute not only to a molecular description of the cellular cholesterol efflux pathway but also to our growing understanding of the mechanisms by which human cholesterol homeostasis is maintained or goes awry.